The present invention relates to an improved method of inhibiting tin whisker growth. More particularly, the present invention relates to a method of implanting ions into tin coatings for the purpose of inhibiting tin whisker growth.
Coating components with tin or tin-containing material have become increasingly important in fabricating electronic circuits, electronic devices, electrical connectors, and other items. Such coatings, for example, protect components from corrosion, provide a chemically stable surface for soldering, and provide good surface electrical contact.
Tin coatings may be applied to metallic substrates by a variety of plating solutions and methods. For example, a tin coating may be formed by immersing an item to be coated in a tin-containing solution. Also, a tin coating may be formed by electrodepositing in solution a tin layer on a substrate. The following references illustrate a variety of methods for applying tin coatings to metallic substrates: U.S. Pat. No. 3,966,564 to Hyner et al.; U.S. Pat. No. 4,049,508 to Morrissey; Japanese Patent Publication No. 55-113894; Russian Patent No. 187,746; the article "Tin-Lead Electrodeposits: The Effect of Impurities in the Solution," by G. W. Cavanaugh et al., Plating, April 1970, pp. 369-371; U.S. Pat. No. 3,663,384 to Lescure; U.S. Pat. No. 3,956,123 to Rosenberg et al.; U.S. Pat. No. 4,029,556 to Monaco et al.; U.S. Pat. No. 4,162,205 to Wilson et al.; U.S. Pat. No. 4,168,223 to Igarashi; U.S. Pat. No. 4,207,148 to Hsu; U.S. Pat. No. 4,347,107 to Teichmann et al.; Japanese Patent Publication No. 54-128947; and Japanese Patent Publication No. 56-65993.
Regardless of the tin-coating method used, it is desirable to form a tin surface that is smooth, that has minimum porosity, that has a relatively constant thickness, that resists interdiffusion with the metal substrate, and that is not brittle. High-purity tin is often used to overcome those problems. However, from high-purity tin can grow spontaneously tin filaments called "tin whiskers". Tin whiskers are undesirable because they, for example, can cause electrical shorts and electrical bridges.
The mechanics of the formation of tin whiskers are not clearly understood. Tin whiskers may form from high-purity tin as soon as one day from the date of application of the tin coating, or they may form several years later. There is speculation in the literature that tin whiskers grow from stress concentration sites. There is also speculation that temperature and humidity affect tin whisker growth. The article "Simultaneous Growth of Whiskers on Tin Coatings: 20 Years of Observation" by S. C. Britton, Transactions of the Institute of Metal Finishing, Vol. 52, 1974, pp. 95-102, discusses the tin whisker problem and offers recommendations for reducing the risk of tin whisker formation.
One approach to lessen the tin whisker problem has been to specify short storage times for tin-coated materials. However, that approach does not fully address or necessarily avoid the problem. Another approach has been to mildly strengthen the tin matrix to prevent the extrusion of tin whiskers. Such strengthening may be accomplished by facilitating diffusion of the metal substrate into the tin, forming, for example, a copper-tin intermetallic compound. The cost of strengthening the tin matrix is a decrease in performance in the final product.
Other approaches to lessen the growth of tin whiskers generally involve the addition of whisker-inhibiting chemicals to the coating solution. U.S. Pat. No. 4,093,466 and No. 4,194,913, both to Davis, disclose the incorporation of both a sulphur containing complexing agent, such as thiourea, and a soluble plumbous salt into a tin immersion bath solution. U.S. Pat. No. 4,263,106 to Kohl discloses electroplating with tin from an acidic aqueous solution containing tin, lead (or alloys of tin and lead), and one or more additions of a class of organic compounds including lactones, lactams, cyclic sulfate esters, cyclic imides, and cyclic oxazolinones. U.S. Pat. No. 4,331,518 to Wilson discloses electroplating with tin from an acid aqueous solution containing tin and a chelate salt comprising an acid bismuth sulfate gluconate. U.S. Pat. No. 4,749,626 to Kadija et al. discloses the incorporation in a tin immersion bath solution of metal salts, including palladium, silver, nickel, iron, cadmium, platinum, gold, indium, ruthenium, and cobalt. U.K. Patent No. 1,167,138 suggests that tin whisker growth may be prevented by reducing the amount of hydrogen absorbed or occluded in the structure of the plating metal. S. M. Arnold, "Repressing the Growing of Tin Whiskers," Plating, January, 1966, pp. 96-99, discloses that a number of metals reduce tin whiskers when present in the tin coating.
The above approaches to the reduction of tin whisker growth have been present for some time. However, they are all limited, in that, each discloses tin plating by only immersion or electroplating.